1. Field of the Invention
The present invention generally relates to ultrasonic fluid flow measurement, and to ultrasonic flowmeters.
The present invention more specifically relates to a disposable acoustic chamber useable with a pre-existing ultrasonic flowmeter in the extracorporeal measurement of the flow of blood, or in the measurement of the flow of other liquids.
2. Description of the Prior Art
2.1 Ultrasonic Fluid Flow Measurement, and Flowmeters
The use of ultrasonic flowmeters of both the transit time and Doppler types to measure the flows of fluids through conduits is well known. Ultrasound in the high frequency range from 100 KHz to 20 MHz, and more commonly in the range from 1 MHz to 20 MHz, is typically used. The ultrasound may be transmitted continuously by a Continuous Wave (CW) ultrasonic flowmeter, or in bursts by a pulsed Doppler ultrasonic flowmeter.
There are two common ultrasonic fluid flow measurement techniques. A transit time technique measures the difference in the transit times of sound projected upstream and downstream through a flowing fluid. This time difference is a function of that component of the vector velocity of fluid flow which is located along the ultrasonic path. The actual ultrasonic path may be diagonally transverse to, or coaxial with, the fluid flow vector. A transit time ultrasonic flowmeter is primarily sensitive to volumetric flow.
In Doppler ultrasonic fluid flow measurement ultrasound is transmitted in either an upstream or a downstream direction through a flowing fluid. The ultrasound is scattered or reflected by particles or bubbles that are suspended in the flowing fluid, and which are moving with the same velocity and direction as does the fluid. A portion of the returned ultrasound is detected, and the Doppler frequency shift of this returned ultrasound is an indication of the flow of the fluid, and more particularly of the velocity spectrum of the flow of the fluid. From this indicated velocity the volume of fluid flow can be calculated when the cross-sectional area of the flow conduit is known.
The detection of Doppler frequency shifted ultrasound sometimes transpires by, and in, the same ultrasonic transducer which initially projected the ultrasound sound in a manner similar to sonar. Sometimes separate transmitting and receiving transducers are used, particularly for continuous wave (CW) Doppler embodiments.
The amount of the Doppler frequency shift undergone by the reflected ultrasound is really a double Doppler frequency shift: one Doppler frequency shift being incurred on the outward path (before reflection) and another Doppler frequency shift being incurred by the reflected signal along its return path. The total amount of the Doppler frequency shift is a function of that component of the vector velocity of fluid flow which is located along both legs of the ultrasonic path. The ultrasonic path is normally at a known angle to the fluid flow vector, typically at a 45.degree. angle. Accordingly, the actual fluid velocity .nu. is inversely proportional to the cosine of the angle between the ultrasonic axis and the flow axis (cos .theta.) and to the frequency of the ultrasound f.sub.o, and is directly proportional to the detected frequency shift (.DELTA.f) and to the velocity of sound (c) in the fluid medium. This may be mathematically stated as: ##EQU1##
Both the transit time and Doppler ultrasonic fluid flow measurement techniques have previously been used to monitor the flow of various fluids through assorted pipes, conduits, and lumens, as well as the flow of blood through blood vessels.
2.2 Electromagnetic Fluid Flow Measurement, and Flowmeters
Meanwhile, it is also known to use flowmeters of the electromagnetic type to measure the flow of fluids, including the flow of blood. In use of an electromagnetic flowmeter for measuring fluid flow an electrically conductive liquid is passed through a conduit--typically either a tube or blood vessel--at right angles to a magnetic field that is typically established by electromagnets. The electrically conductive flowing fluid serves as a moving conductor in the magnetic field, and, by the laws of physics, induces an electromagnetic force (EMF) in a direction perpendicular to the applied magnetic field. This EMF can be measured as a voltage differential between electrodes that are positioned diametrically oppositely across the conduit along a diameter that extends perpendicularly to the lines of magnetic flux. The voltage generated is proportional to the volume rate of blood flow. Blood may suitably serve as an electrically conductive fluid.
2.3 Challenges of Extracorporeal Blood Flow Measurement, and Previous Electromagnetic Flowmeters Used in Extracorporeal Blood Flow Measurement
The flow of blood in a live human patient must be measured and/or monitored upon certain occasions, such as during the connection of an artificial heart and lung machine into the patient's circulatory system. The patient's blood flow is most conveniently and safely so measured extracorporeally (outside the body). Historically, and although acoustic flowmeters have been known for over forty years, the extracorporeal measurement of blood flow has typically transpired by use of electrical equipments, typically electromagnetic flowmeters. The use of electromagnetic flowmeters has possibly been related to the frequent presence of other electrical, electronic, and electromagnetic equipments during surgical procedures, such as open heart surgery, that require the extracorporeal measurement of blood flow and blood circulation. More recently ultrasonic flowmeters have also come into use for the extracorporeal measurement of blood flow.
Progress in facilitating the somewhat cumbersome connection of a patient for extracorporeal blood flow measurement has been made only for electromagnetic blood flow measurement, and then only to a limited extent. Before the invention of the partially-disposable in-line electromagnetic flow measurement transducer shown and described in U.S. Pat. No. 4,195,515 to Smoll, the required sensor assembly of an electromagnetic flowmeter used to perform an extracorporeal blood flow measurement or monitoring was normally assembled as one composite unit. The assembled sensor assembly typically included a precision-aligned (i) flow tube, (ii) electrodes and (iii) magnet. The assembled, unitary, sensor assembly was expensive and difficult to sterilize after each use. Separate sensor assemblies were required for each flow channel, or flow point, to be measured. If measurements were to transpire in differently sized flow lines then differently sized sensor assemblies were required.
The in line electromagnetic flow measurement transducer shown and described in U.S. Pat. No. 4,195,515 to Smoll divided the previous composite electromagnetic flowmeter sensor assembly into two separate, physically and electrically plug-connected, parts. One part was a tubular member having electrode sensors. The tubular member readily and easily flow connected to elastomeric surgical tubes through nozzle features. It was typically so inexpensive as to be disposable. Alternatively, the tubular member could be readily and easily interchanged and sterilized by conventional techniques.
The other part of Smoll's composite sensor assembly contained the relatively more expensive magnet used to generate the required magnetic field. When the two parts were physically and electrically quick-connected to each other by plugging, they formed the complete sensor assembly suitable for use with an electromagnetic flowmeter. A number of differently-sized first-part tubular members could each be used with the same second-part magnet member as best suited flow measurements taken at different points, and/or in differently sized flow lines.
2.4 Difficulties With the Use of In Line Electromagnetic Flow Transducers, and Electromagnetic Flowmeters, During Extracorporeal Blood Flow Measurement
A first problem with the existing use of electromagnetic flowmeters in extracorporeal blood flow measurement is that an electromagnetic flowmeter commonly incurs an offset voltage, i.e. some voltage may be generated between the electrodes even when no fluid is flowing. This offset voltage typically drifts over time. To account for this drifting offset voltage, an electromagnetic flowmeter is typically zeroed and re-zeroed, by adjustment of its scale or otherwise, so as to properly read a zero flow when no flow is, in fact, present.
This offset, and necessary zeroing and re-zeroing, presents two problems during extracorporeal blood flow measurement. First, it is seldom convenient to stop, or re-stop, blood flow to a patient for which blood flow is being measured, or monitored, in order to zero, or re-zero, the flowmeter. Second, the offset may drift in an unpredictable manner due to uncontrolled changes in the environment of, and between, the electrodes; most notably changes in the conductance of the blood and/or changes in electrode impedance due to chemical action over time.
Previous electromagnetic blood flow measurements have generally been typically sufficiently accurate so that the general adequacy, and the continuance, of the circulation of blood in an patient may be monitored. However, because of a drift in the precision of measurement, electromagnetic blood flow measurement has generally lacked such accuracy as would permit minor trends and/or perturbations in blood flow to be observed. Because these trends and/or perturbations, howsoever minor, may be important relative to the surgical procedures being performed, it is best if they are promptly, clearly and unambiguously detected.
By comparison, ultrasonic flowmeters do not suffer from these limitations. Offsets in ultrasonic flowmeters are much lower in magnitude, and ultrasonic flowmeter are more stable with time than are electromagnetic flowmeters. Any such calibration as needs be performed on an ultrasonic flowmeter requires no alteration, nor any bypass, of the blood flow.
A second problem with the use of electromagnetic flowmeters in extracorporeal blood flow measurement is that such flowmeters require direct, intimate, electrically-conducting contact between metal electrodes and the fluid blood. This contact is undesirable in that reaction chemistry, contamination, flow path disruption or perturbation, and/or blood clotting may occur.
Conversely, and by comparison, there need be no direct contact between the transducers of an ultrasonic flowmeter and the fluid, or blood, for which flow is measured. Medical grade plumbing such as surgical hose typically serves as a conduit for the flow of blood outside a patient's body. The walls of the conduit are typically seamless and smooth, and without substantial chemical or physical differentiation from region to region. The conduit walls are typically non-thrombogenic, and do not induce blood clotting.
Ultrasonic transducers are disposed to the exterior of the conduit, and transmit sound through the conduit walls as well as through the flowing fluid blood. In this location the transducers never come into direct contact with the blood, nor with anything that is within the blood. The conduit through which blood flows during ultrasonic blood flow measurement may be a blood vessel itself as well as, typically, plastic surgical tubing which is flow-connected to a blood vessel.
2.5 A Previous Attempt to Use An Ultrasonic Flowmeter During Extracorporeal Blood Flow Measurement
With increasing use of ultrasonic flowmeters of both the transit time and Doppler types for measuring blood flow within the body of a live patient or animal (i.e., in vivo), it has previously been contemplated to also use ultrasonic flowmeters for measuring the flow of blood outside of the patient's or the animal's body. In order to so measure extracorporeal blood flow, (i) one or more ultrasonic transducers must be ultrasonically coupled to a flow of blood outside the patient's body, and (ii) an ultrasonic path must be established and defined through the flowing blood.
One previous system for realizing both requirements is the SNAP-ON FLOW MEASUREMENT SYSTEM of Lynnworth described in U.S. Pat. No. 5,179,862. Although Lynnworth is primarily concerned with industrial fluid flow measurement applications, one embodiment of his system contemplates a conduit, normally a surgical hose, for channeling a flow of blood. The surgical hose is attached to a support block that also serves to mount one of more ultrasonic transducers. The transducers are held in positions so that they both launch and receive ultrasound signals along a precisely defined path through the flowing fluid. The path may be either be axial along the conduit and along the path of the flowing fluid therein--as is common for smaller conduits channeling lessor blood flows--or--as is common for larger conduits channeling more voluminous blood flows--obliquely across the conduit. The oblique paths include paths crossing the conduit more than one time in a zig-zag fashion with multiple acoustic reflections.
In the Lynnworth system channels of predetermined configuration within the support block--which block and channels may both be physically sizable--hold the conduit in acoustic contact and precise alignment with ultrasonic transducers that are located externally to the conduit. This acoustic coupling is difficult and laborious to establish, and occasionally of poor quality. The setup of the conduit (the surgical hose) and the ultrasonic transducers normally requires the attention of a skilled technician, especially if critical reliance is to be made on the flow measurement results during surgery.
In the Lynnworth system ultrasonic waves must, by definition, penetrate the walls of the conduit (the surgical hose) at least twice (once in each of the transmit and receive directions), thereby presenting an undesirable source of reduction in ultrasonic signal due to attenuation and refraction.
Accordingly, it would be desirable if some improvement could be made to the easy, inexpensive, accurate, safe, and convenient use of an ultrasonic flowmeter (of either the transit time of Doppler types) during extracorporeal blood flow measurement.